Mudder":3pbqqdxr said:
My point was that if the bias is "fixed" then the design allows for various tubes of the same type to be used without worrying about bias much.
That's unfortunately not the case. There is no "fixed bias" circuit which makes it safe for various tubes--even of the same type--unless the bias voltage is set to a very skewed value such that it biases most tubes "cold".
Getting the term "fixed bias" confused with a bias supply circuit which has no adjustment control (which may even include "cathode bias"), leads to problems understanding what a person should do when changing tubes.
"Fixed bias" means the voltage supplied to the tubes can remain (mostly, realistically) constant irrespective of how the tubes are operating in the circuit; the bias voltage comes from a separate circuit outside the operation of the tube, set to a specific voltage value. This voltage value is set to be appropriate for the tubes in the design at the time. There is usually a resistor to help establish the correct value of voltage, and that resistor can be variable (a potentiometer or varistor) for adjustment. That particular bias voltage can't be counted on to be appropriate for other tubes unless they've been tested to coincidentally bias the same under those exact conditions, such was when you buy Mesa-branded replacement tubes for your Mesa amplifier which has no bias pot. "Fixed bias" doesn't imply that the bias can or cannot be adjusted. The term only refers to the voltage, which can be set to a specific constant (fixed) value. If anything "fixed bias" implies that the bias voltage should be adjusted for different tubes (even the same brand/type) since the bias voltage is coming from a relatively independent circuit (independent of the operation of the tubes). That is of course with the exception of vendors selling tubes tested to bias appropriately under the conditions the amplifier provides (such as buying Mesa-branded tubes for stock Mesa amps).
With "cathode bias", the operation of the tube generally set the bias. Even though there has to be an appropriate cathode resistor in place, usually it can be ballparked for its value for the appropriate bias voltage various tubes will require in the circuit. If a tube draws more or less current, the bias voltage changes based on the current the tube is drawing and a suitable balance establishes. This isn't perfect for all tubes of course but it's a lot more "plug and play" than fixed bias, by a long shot.
"Fixed bias" can result in widely varying current draw from tube to tube if the voltage isn't adjusted/set properly for the different tubes. Even in a stock 5150 you can't count on every 6L6 biasing in a normal or "cold" range. It may be likely that most 6L6 will bias in a safe range but it's still best to check.
As for "adjustable fixed bias" and "non-adjustable fixed bias", unfortunately most people try to shorten this to save time explaining things and usually get it wrong in the process.
Also, whereas "fixed bias" is a term used to describe a sort of bias circuit, "adjustable" and "non-adjustable" usually isn't considered part of the circuit's normal description. The difference is too slight, relying only on whether a resistor or varistor/potentiometer is in the circuit in a particular place. That's why you don't see it mentioned much in regards to "what sort of fixed bias circuit" is in an amp. Some manufacturers presume you know there's an adjustment pot (most makers of fixed-bias amps), some presume you know there isn't (such as Mesa). Add to that the confusion which arises from people used to 1) using replacement tubes which bias the same, from a particular vendor who tests for this in specific amps, and 2) replacing tubes and not adjusting the bias, and "everything was fine" (in other words, things coincidentally were ok, but the person goes on to presume it's always going to work out the same). This is why the nature of "fixed bias" is so misunderstood.
Cheers. 